The present invention relates generally to electrochemical energy storage devices, and particularly to multiple-cell secondary batteries having a common electrolyte.
The present invention is directed to minimizing the effect of parasitic currents in secondary batteries having a plurality of cells connected electrically in series and a common electrolyte in communication with the cells. Parasitic currents are those electrical currents which flow in the conductive paths created by the network of electrolytic connections linking the cells. In the case of batteries having a circulating electrolyte, these electrolytic connections include conduits for supplying electrolyte to the cells from a reservoir, as well as conduits for returning the electrolyte to the reservoir from the cells. The conduits act as shunt resistors connected across the battery cells, whose effect is to cause a limited current to flow discharging the cells. This parasitic discharge current will oppose the direction of the charge current during the charging of the battery, and thereby reduce the amount of the charge current utilized to charge the battery cells. The parasitic discharge current will also flow during the discharging of the battery, and even when the battery is not connected to a load. In fact, the parasitic discharge currents will only cease to flow when the battery is completely discharged, or the battery is in an open circuit condition and one or both reactant species is denied access to the electrodes or there is insufficient electrolyte in the conduits to create the necessary conductive paths. Accordingly, parasitic currents are considered to be highly undesirable, and many attempts have been made to reduce or eliminate parasitic currents (also known as shunt currents) in multiple-cell batteries. Reference may be had to U.S. Pat. No. 4,197,169, entitled "Shunt Current Elimination and Device", issued on Apr. 8, 1980 to Zahn et al, for a discussion of the many and various attempts made. However, as stated above, the present invention is not directed to reducing or eliminating parasitic currents per se, but rather to minimizing the effect of the parasitic currents. Specifically, the present ivention is directed to minimizing cell imbalances in multiple-cell batteries which result due to the flow of parasitic currents over a charge/discharge cycle.
The term cell imbalance generally refers to performance differences between the cells in the battery. There may be, of course, many causes and reasons for variations in performance between the cells, such as manufacturing tolerances or assembling procedures. While these and other similar causes create cell imbalances which are random in nature, the cell imbalance resulting from parasitic currents follow a predetermined pattern and the magnitude of these imbalances are predictable in nature. Briefly, the slow discharge of the cells caused by the flow of parasitic currents is not uniform with respect to each of the cells in the battery. Rather, the parasitic discharge current flowing through each cell is dependent upon its relative position in the battery, with the cells at the ends of the battery having a lower parasitic discharge current than the cells in the center of the battery. For example, if the battery is comprised of 60 cells connected electrically in series, the end cells (Nos. 1 and 60) will have a lower parasitic discharge current than the center cells (Nos. 30 and 31). As will be more fully appreciated from the detailed description below, the parasitic discharge current gradually increases from the end cells to the central cells, such that the end cells (Nos. 1 and 60) will experience the lowest parasitic discharge current and the center cells (Nos. 30 and 31) will experience the highest parasitic discharge current.
During the charging of the battery, the parasitic discharge current will oppose the direction of the charge current, and thereby reduce the amount of the charge current utilized to charge the battery cells. Accordingly, the center cells of the battery will be charging at a lower rate than the end cells. During the discharging of the battery, the parasitic discharge current will have the same direction as the discharge current. The parasitic discharge current will then be added to the discharge current, and thereby increase the total amount of current utilized to discharge the battery cells. Accordingly, the center cells of the battery will be discharging at a higher rate than the end cells. Thus, over the charge/discharge cycle the cells which were charged at a lower rate will be discharged at a higher rate, and the cells which were charged at a higher rate will be discharged at a lower rate. This imbalance will cause an uneven discharge of the battery cells, such that the center cells will be discharged before the end cells. This will not only reduce the electrochemical efficiency achievable for a single charge/discharge cycle, but the cell imbalance will also become more pronounced during subsequent cycles unless the battery is fully discharged each cycle.
Accordingly, it is a principal object of the present invention to minimize cell imbalances due to parasitic currents over a charge/discharge cycle in a secondary battery having a plurality of cells connected electrically in series and a common electrolyte in communication with the cells.
The present invention provides a method of minimizing cell imbalances whereby the battery is in effect divided into two groups of cells, with the cells in each group connected electrically in series. The battery is charged with the two groups of cells connected electrically in series, and then discharged with the two groups of cells reconnected electrically in series in an inverted sequence. Accordingly, in the 60 cell battery example described above, the battery would be charged with cell Nos. 30 and 31 connected electrically together, and cells 1 and 60 connected across a suitable d.c. power supply. Then the battery would be discharged in the inverted sequence, where cell Nos. 1 and 60 are connected electrically together and cell Nos. 30 and 31 are connected across a suitable loaded. Thus, it will be seen that the cells which experienced the greatest parasitic discharge current losses during charging, will also experience the least parasitic discharge current increases during discharging. Similarly, the cells which experienced the least parasitic discharge current losses during charging, will also experience the greatest parasitic discharge current increases during discharging.
Additional advantages and features of the present invention will become apparent from a reading of the detailed description of the preferred embodiments which makes reference to the following set of drawings in which: